higher than usual RPM

[PreludeMOFO]

ive noticed that with in the last month or so my tachometer is reading higher than normal. It seems that for whatever the car is doing (idle, street driving, freeway driving, etc...) the RPM's are running a good 1000 more than usual.

also when im tranfering from 4th to 5th gear and going above 50 miles an hour, the tach wil jump up and down from say 4000 to 5000 RPM. also i used to idle round 1000 RPM but am now 2000 RPM.

With all of these i do not hear a noticable increase in sound from the car.

What do u think is causing this? Bad sensor? running too rich?

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1994 Honda Prelude VTEC
STOCK for now